Progress in the structural and functional characterization of kinetochores.

Kinetochores are macromolecular complexes built on a specialized chromatin domain called the centromere. Kinetochores provide a site of attachment for spindle microtubules during mitosis. They also control a cell cycle checkpoint, the spindle assembly checkpoint, which coordinates mitotic exit with the completion of chromosome alignment on the mitotic spindle. Correct kinetochore operation is therefore indispensable for accurate chromosome segregation. With multiple copies of at least 30 structural core subunits and a myriad of regulatory subunits, kinetochores are among the largest known macromolecular machines. Biochemical reconstitution and structural analysis, together with functional studies, are bringing to light the organizational principles of these complex and fascinating structures. We summarize recent work and identify a few challenges for future work.